1. Field of the Invention
The present invention relates to a reforming catalyst for fuel cells adapted for use in decomposition of hydrocarbons or alcohols in a molten carbonate type fuel cell power generating system, and more particularly, to a reforming catalyst having an extended activity life.
2. Description of the Prior Art
FIG. 2 schematically shows an example of a power generating system of an externally reforming type having a reformer provided outside a molten carbonate fuel cell, as disclosed in United States GRI report FCR-3522-2. In FIG. 2, the fuel cell power generating system illustrated comprises: a reformer 1 having a tubular reforming reaction portion 1a and a heater portion 1b and adapted to reform a fuel gas including hydrocarbons or alcohols into one including hydrogen and carbon monoxide; a temperature controlling device 2 for controlling the temperature of the fuel gas reformed by the reformer 1; a molten carbonate fuel cell 3 adapted to be supplied with the reformed fuel gas from the reformer 1, to generate electric power; a temperature and humidity controlling device 4 adapted to remove excess moisture from the fuel gas discharged from the fuel cell 3; an air supplying device 5 for supplying air to the heater portion 1b of the reformer 1; and a heat exchanger 6 adapted to take up heat from a part of the oxidant gas which is discharged from the outlet side of the fuel cell 3 and recirculates through the heat exchanger 6 into the inlet side of the fuel cell 3.
In operation, as seen from FIG. 2, a fuel gas including hydrocarbons or alcohols together with a part of the reacted fuel gas discharged from the molten carbonate fuel cell 3 is supplied to the tubular reforming reaction portion 1a of the reformer 1, wherein the hydrocarbons or alcohols in the fuel gas thus supplied are reformed under the action of an appropriate reforming catalyst contained in the reforming reaction portion 1a to produce hydrogen and carbon monoxide, as shown in the following formulae. ##EQU1##
The reaction heat necessary for the above reforming reactions is obtained by combustion of the remaining part of the fuel gas discharged from the molten carbonate fuel cell 3 in the heater portion 1b.
The fuel gas containing hydrogen and carbon monoxide thus produced is introduced from the reformer 1 into the temperature controlling device 2 wherein the temperature of the fuel gas is appropriately controlled. Thereafter, the temperature-controlled fuel gas is fed to the fuel-gas electrode side of the molten carbonate fuel cell 3.
The molten carbonate fuel cell 3 normally operates at a temperature of 650.degree. C., and the following reactions take place at the fuel gas electrode and the oxidant gas electrode of the fuel cell 3, respectively:
at the fuel gas electrode; ##EQU2##
at the oxidant gas electrode; ##EQU3##
Through the above chemical and electrochemical reactions, the chemical energy of the fuel gas is converted into electrical energy and by-producing thermal energy.
The fuel gas discharged from the molten carbonate fuel cell 3 contains a lot of water vapour as a result of the above reaction (4), and a portion of the fuel gas is returned to the tubular reforming reaction portion 1a of the reformer 1 so as to supply a required amount of water vapour for the above-described reforming reactions (1), (2) and (3).
The remaining portion of the fuel gas discharged from the fuel cell 3, having excess water vapour removed and then appropriately heated by the temperature and humidity controlling device 4, is fed, together with the air from the air supplying device 5, to the heater portion 1b of the reformer 1 wherein the fuel gas reacts with oxygen in the air to combust thus producing combustion heat, almost all of which is given to the reforming reaction portion 1a of the reformer 1 as reaction heat necessary for the reforming reactions (1), (2) and (3).
The fuel gas almost completely oxidized in the heater portion 1b of the reformer 1 is fed, together with the air from the air supplying device 5, to the oxidant gas electrode of the molten carbonate fuel cell 3.
A portion of the oxidant gas discharged from the outlet side of the fuel cell 3 is returned to the inlet side of the fuel cell 3 by way of the heat exchanger 6 so that surplus heat is removed from the oxidant gas and taken out to the outside by means of the heat exchanger 6, and the fuel gas thus removed of surplus heat serves to cool the molten carbonate fuel cell 3.
In this connection, it is to be noted that the conventional reforming catalyst filled in the tubular reforming reaction portion 1a of the reformer 1 is, for example, composed of a substance having catalytic activity such as, for example, nickel carried on a carrier such as a porous inorganic substance having a fine porous structure such as Al.sub.2 O.sub.3, LiAlO.sub.2, CeO.sub.2, MgO and the like. The activity of such a reforming catalyst usually more or less changes or reduces with lapse of time, but if such a change is great, problems will arise affecting the steady-state operation of the power generating system. The general causes for such activity reduction are listed below:
(A) The carrier changes chemically or structurally to alter the fine porous structure thereof:
(B) The catalyst is poisoned:
(C) The fine pores in the catalyst are clogged, and the activity points of the catalyst are covered by other substances.
In the power generating system as illustrated in FIG. 2, the water vapour contained in the fuel gas discharged from the molten carbonate fuel cell 3 is effectively utilized as the water vapour necessary for the above reforming reactions (1), (2) and (3), which is a characteristic feature of the system arrangement of FIG. 2. However, the fuel gas supplied to the tubular reforming reaction portion 1a contains electrolytes such as, for example, K.sub.2 CO.sub.3 and Li.sub.2 CO.sub.3 evaporized from the electrolyte layer of the molten carbonate fuel cell 3, and the substances such as, for example, KOH and LiOH produced from the electrolytes. These electrolytes and substances produced therefrom serve to promote the reduction in activity of the reforming catalyst. More specifically, the electrolytes or the substances produced therefrom and the carrier chemically react with each other to alter (1) the composition and the fine porous structure of the carrier and (2) produce condensation in the fine pores of the reforming catalyst, and/or (3) cover the active points of the catalyst, thereby promoting activity reduction of the catalyst.
Accordingly, in the system arrangement as illustrated in FIG. 2, the activity reduction of the reforming catalyst is great, and hence such a power generating system is advantageous in terms of effective utilization of the water vapour generated in the fuel cell, but disadvantageous from the viewpoint of the steady-state operation of the power generating system.
On the other hand, FIG. 3 is a perspective view in vertical cross section, showing a molten carbonate fuel cell power generating system of an internally reforming type having a reformer provided inside a molten carbonate fuel cell. In FIG. 3, the fuel cell power generating system illustrated comprises a fuel cell unit including an electrolyte layer 7; a fuel-gas-side electrode 8; a collector panel 10 for supporting the fuel-gas-side electrode 8 or the oxidant-gas-side electrode 9; separator panels impervious to air and adapted to electrically connect a plurality of fuel cell units in a series relation with each other and form gas chambers; gas-passage forming corrugated panels 12 located in the respective gas chambers to form reaction gas passages therein and adapted to assist the electrical connection between the fuel cell units; and a reforming catalyst 13 located in the fuel-gas-side of the respective gas chambers. In FIG. 3, arrow A designates the flow direction of the fuel gas, and arrow B the flow direction of the oxidant gas.
In FIG. 3, the reforming catalyst 13, similar to that employed in the prior art fuel cell power generating system as illustrated in FIG. 2, is inserted in the fuel gas passages defined by the respective gas-flow-passage forming corrugated panels 12. In a molten carbonate fuel cell of the internally reforming type, the reforming catalyst 13 is disposed at a location nearer the electrolyte layer 7 containing an electrolyte and the fuel gas electrode 8 than it is in an externally reforming type fuel cell, so that the reduction in activity of the reforming catalysts 13 due to adverse influences from the electrolyte is far greater than in the case of a power generating system of the externally reforming type shown in FIG. 2. The effective life of an existing molten carbonate fuel cell of the internally reforming type greatly depends on the activity of the reforming catalysts 13. Presently, the effective life of these fuel cells ranges from about several thousand to about ten thousand hours.
In this connection, it is to be noted that a general process for producing a conventional reforming catalyst is described in British Pat. No. 1,182,829.
As apparent from the foregoing, in order to enable the molten carbonate fuel cell power generating system to operate in a stable manner for an extended period of time, it is necessary to develop a novel reforming catalyst, the activity level of which is not lowered due to the vapour of the electrolyte.
The above-described conventional reforming catalyst, however, has the problem that the catalytic activity of the reforming catalyst will be lowered due to contact thereof with an electrolyte generated by the fuel cell or substances produced by the electrolyte, so that the fuel cell power generating system employing such a conventional reforming catalyst can not operate in a stable manner for a long period of time because of degradation in activity of the reforming catalyst.